Although some excellent articles about Lyell's work have been published, they do not explicitly deal with Lyell's biogeographical conceptions. The purpose of this paper is to analyse Lyell's biogeographical model in terms of its own internal structure. Lyell tried to explain the distribution of organisms by appealing to a real cause (climate). However, he was aware that environmental conditions were clearly insufficient to explain the existence of biogeographical regions. Lyell's adherence to ecological determinism generated strong tensions within his biogeographical model. He shifted from granting a secondary weight to dispersal to assigning it a major role. By doing so, Lyell was led into an evident contradiction. A permanent tension in Lyell's ideas was generated by the prevalent explanatory pattern of his time. The explanatory model based on laws did not produce satisfactory results in biology because it did not deal with historical processes. We may conclude that the knowledge of organic distribution interested Lyell as long as it could be explained by the uniformitarian principles of his geological system. The importance of the second volume of the Principles of geology lies in its ample and systematic argumentation about the geographical distribution of organisms. Lyell established, independently from any theory about organic change, the first version of dispersalist biogeography.